The objective of this project is to study the differentiation of myofibrillar proteins in skeletal and cardiac muscle of embryonic, postpartum, adult and aged animals. During the initial stage of differentiation muscle fibers do not differ in their physiological or biochemical properties. These differences appear at a later stage, which in many mammals is not complete until a few weeks after birth. Adult mammalian muscles of different functional type show well defined differences in their phsyiological, biochemical and histochemical characteristics. Our first objective is to explore the differentiation in embryonic and post-partum developmental stages and to investigate what changes occur in muscles of aged animals. We believe, a) that the characteristic properties of slow-contracting muscle fibers develop an adaptive response to the continuous low-frequency activity which is imposed on them by motorneurones and b) that the presence of this pattern of activity is a necessary condition for the retention of these properties. The proposed experiments will deal with the various aspects of the determination and maintenance of distinct muscle types (slow, fast) in terms of changes in activity pattern, neural influence, development and aging. BIBLIOGRAPHIC REFERENCES: Sreter, F.A., Balint, M. and Gergely, J., Structural and Functional Changes of Myosin During Development - Comparison with Adult Fast, Slow and Cardiac Myosin, Developmental Biol. 46, 317, 1975. Sreter, F.A., Luff, A.L. and Gergely, J., The Effect of Cross-Reinnervation on Physiological Parameters and on Properties of Myosin and Sarcoplasmic Reticulum of Fast and Slow Muscles of the Rabbit, J. Gen. Physiol. 66, 811, 1975.